Rotating a mirrored object by 90 degrees doesn't work as expected
Make sure that the Pivot Point is located in the centre of the selected bar. 

To ensure that, hit Ctrl+Space to make it visible. If it's already, than skip this. You can also see that using this button:



Then either hit Ctrl + , (it will snap the pivot point to the median point, that is what you want) or use this menu:



In this case it is appropriate to select any of these opitons except 3D Cursor.

Then select your bar, hit R, type some number and release left mose button or hit Space.

Option Clipping in mirror modifier prevents vertices that are on the mirror plane from being moved from it. You would really like to use it if creating any characters, cars etc, when you wouldn't like vertices on the mirror plane being doubled. See this for more info.